1. Field of the Invention
The present invention relates to a method of spot welding aluminum alloy workpieces, and more particularly to such a method which permits a significant increase in the number of welding cycles that can be performed by welding electrodes in a continuous spot-welding operation on the aluminum alloy workpieces.
2. Discussion of the Related Art
As one of resistance welding methods for aluminum alloy materials, there is known a spot-welding method which includes the steps of: a) superposing on each other two aluminum alloy sheets to be joined together; b) pressing a pair of electrodes onto the aluminum alloy sheets, such that the electrodes are opposed to each other via the sheets; and c) applying an electric current to the electrodes to melt faying or abutting surfaces of the sheets interposed between the electrodes, due to resistance heat, to thereby form a nugget and join the two sheets together. Such a spot-welding method may be practiced according to JIS (Japanese Industrial Standard) Z-3234 or ISO 5182, which specifies that electrodes formed of Cu--Cr alloy, Cu--Cr--Zr alloy or the like are suitably used for resistance spot welding, in terms of their mechanical properties, electrical conductivity and others.
When the above-described known electrodes are used for effecting a continuous spot welding operation in which aluminum alloy workpieces are welded at multiple spots one after another in repetitive welding cycles, however, the tips of the electrodes are likely to be worn off during an initial period of the intended continuous spot welding operation, that is, when only a relatively small number of welding cycles are accomplished. The wear of the electrodes results in reduced strength of welded portions of the workpieces. Namely, the use of the above known electrodes involves some problems, such as a relatively short service life of the electrodes, and a comparatively small number of welding cycles which can be continuously effected by the electrodes.
Aluminum alloy sheets, when used for automobile vehicle body panels, for example, are spot welded after the sheets are formed in desired shape by pressing. In many cases, the aluminum alloy sheets are stained with lubricating oil adhering thereto during the pressing, before proceeding to the next spot-welding step. The presence of the lubricating oil on the sheets results in a further shortened service life of the electrodes, and a further reduced number of welding cycles which can be continuously effected by the electrodes.